Merge #1071

1071: Allow transcribing of zero nodes in certain cases r=CohenArthur a=CohenArthur

When expanding AST fragments containing multiple nodes, we must be aware
that some cases allow expanding zero or more nodes. Any macro
transcription that gets parsed as many nodes (ie any transcriber function that calls `parse_many`) needs to be able to parse zero of those nodes and still get expanded properly (basically, removed).

Previously, this would cause a failure to lower the macro invocation which would remain as a child instead of getting stripped/erased.



Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>

Author: bors[bot]

gcc/rust/ast/rust-ast.h

@@ -1871,13 +1871,12 @@ class ASTFragment
     return *this;
   }
 
-  static ASTFragment create_empty () { return ASTFragment ({}); }
   static ASTFragment create_error () { return ASTFragment ({}, true); }
 
   std::vector<SingleASTNode> &get_nodes () { return nodes; }
   bool is_error () const { return fragment_is_error; }
 
-  bool should_expand () const { return !is_error () && !nodes.empty (); }
+  bool should_expand () const { return !is_error (); }
 
   std::unique_ptr<Expr> take_expression_fragment ()
   {

gcc/rust/ast/rust-macro.h

@@ -469,7 +469,7 @@ class MacroRulesDefinition : public MacroItem
   static ASTFragment dummy_builtin (Location, MacroInvocData &)
   {
     gcc_unreachable ();
-    return ASTFragment::create_empty ();
+    return ASTFragment::create_error ();
   }
 
   /* NOTE: in rustc, macro definitions are considered (and parsed as) a type

gcc/rust/expand/rust-macro-builtins.cc

@@ -141,7 +141,7 @@ MacroBuiltin::assert (Location invoc_locus, AST::MacroInvocData &invoc)
 {
   rust_debug ("assert!() called");
 
-  return AST::ASTFragment::create_empty ();
+  return AST::ASTFragment::create_error ();
 }
 
 AST::ASTFragment

gcc/rust/expand/rust-macro-expand.cc

@@ -195,7 +195,7 @@ MacroExpander::expand_invoc (AST::MacroInvocation &invoc)
   bool ok = mappings->lookup_macro_def (resolved_node, &rules_def);
   rust_assert (ok);
 
-  auto fragment = AST::ASTFragment::create_empty ();
+  auto fragment = AST::ASTFragment::create_error ();
 
   if (rules_def->is_builtin ())
     fragment
@@ -236,7 +236,7 @@ MacroExpander::expand_invoc_semi (AST::MacroInvocation &invoc)
   bool ok = mappings->lookup_macro_def (resolved_node, &rules_def);
   rust_assert (ok);
 
-  auto fragment = AST::ASTFragment::create_empty ();
+  auto fragment = AST::ASTFragment::create_error ();
 
   if (rules_def->is_builtin ())
     fragment
@@ -781,7 +781,7 @@ MacroExpander::match_repetition (Parser<MacroInvocLexer> &parser,
 /**
  * Helper function to refactor calling a parsing function 0 or more times
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 parse_many (Parser<MacroInvocLexer> &parser, TokenId &delimiter,
 	    std::function<AST::SingleASTNode ()> parse_fn)
 {
@@ -795,7 +795,7 @@ parse_many (Parser<MacroInvocLexer> &parser, TokenId &delimiter,
       nodes.emplace_back (std::move (node));
     }
 
-  return nodes;
+  return AST::ASTFragment (std::move (nodes));
 }
 
 /**
@@ -804,7 +804,7 @@ parse_many (Parser<MacroInvocLexer> &parser, TokenId &delimiter,
  * @param parser Parser to extract items from
  * @param delimiter Id of the token on which parsing should stop
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_many_items (Parser<MacroInvocLexer> &parser, TokenId &delimiter)
 {
   return parse_many (parser, delimiter, [&parser] () {
@@ -819,7 +819,7 @@ transcribe_many_items (Parser<MacroInvocLexer> &parser, TokenId &delimiter)
  * @param parser Parser to extract items from
  * @param delimiter Id of the token on which parsing should stop
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_many_ext (Parser<MacroInvocLexer> &parser, TokenId &delimiter)
 {
   return parse_many (parser, delimiter, [&parser] () {
@@ -834,7 +834,7 @@ transcribe_many_ext (Parser<MacroInvocLexer> &parser, TokenId &delimiter)
  * @param parser Parser to extract items from
  * @param delimiter Id of the token on which parsing should stop
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_many_trait_items (Parser<MacroInvocLexer> &parser,
 			     TokenId &delimiter)
 {
@@ -850,7 +850,7 @@ transcribe_many_trait_items (Parser<MacroInvocLexer> &parser,
  * @param parser Parser to extract items from
  * @param delimiter Id of the token on which parsing should stop
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_many_impl_items (Parser<MacroInvocLexer> &parser, TokenId &delimiter)
 {
   return parse_many (parser, delimiter, [&parser] () {
@@ -865,7 +865,7 @@ transcribe_many_impl_items (Parser<MacroInvocLexer> &parser, TokenId &delimiter)
  * @param parser Parser to extract items from
  * @param delimiter Id of the token on which parsing should stop
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_many_trait_impl_items (Parser<MacroInvocLexer> &parser,
 				  TokenId &delimiter)
 {
@@ -881,7 +881,7 @@ transcribe_many_trait_impl_items (Parser<MacroInvocLexer> &parser,
  * @param parser Parser to extract statements from
  * @param delimiter Id of the token on which parsing should stop
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_many_stmts (Parser<MacroInvocLexer> &parser, TokenId &delimiter)
 {
   auto restrictions = ParseRestrictions ();
@@ -901,28 +901,28 @@ transcribe_many_stmts (Parser<MacroInvocLexer> &parser, TokenId &delimiter)
  *
  * @param parser Parser to extract statements from
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_expression (Parser<MacroInvocLexer> &parser)
 {
   auto expr = parser.parse_expr ();
 
-  return {AST::SingleASTNode (std::move (expr))};
+  return AST::ASTFragment ({std::move (expr)});
 }
 
 /**
  * Transcribe one type from a macro invocation
  *
  * @param parser Parser to extract statements from
  */
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_type (Parser<MacroInvocLexer> &parser)
 {
-  auto expr = parser.parse_type ();
+  auto type = parser.parse_type ();
 
-  return {AST::SingleASTNode (std::move (expr))};
+  return AST::ASTFragment ({std::move (type)});
 }
 
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_on_delimiter (Parser<MacroInvocLexer> &parser, bool semicolon,
 			 AST::DelimType delimiter, TokenId last_token_id)
 {
@@ -932,7 +932,7 @@ transcribe_on_delimiter (Parser<MacroInvocLexer> &parser, bool semicolon,
     return transcribe_expression (parser);
 } // namespace Rust
 
-static std::vector<AST::SingleASTNode>
+static AST::ASTFragment
 transcribe_context (MacroExpander::ContextType ctx,
 		    Parser<MacroInvocLexer> &parser, bool semicolon,
 		    AST::DelimType delimiter, TokenId last_token_id)
@@ -1049,7 +1049,7 @@ MacroExpander::transcribe_rule (
   //   as a statement (either via ExpressionStatement or
   //   MacroInvocationWithSemi)
 
-  auto nodes
+  auto fragment
     = transcribe_context (ctx, parser, semicolon,
 			  invoc_token_tree.get_delim_type (), last_token_id);
 
@@ -1072,6 +1072,6 @@ MacroExpander::transcribe_rule (
 		     "tokens here and after are unparsed");
     }
 
-  return AST::ASTFragment (std::move (nodes));
+  return fragment;
 }
 } // namespace Rust

gcc/rust/expand/rust-macro-expand.h

@@ -200,7 +200,7 @@ struct MacroExpander
   MacroExpander (AST::Crate &crate, ExpansionCfg cfg, Session &session)
     : cfg (cfg), crate (crate), session (session),
       sub_stack (SubstitutionScope ()),
-      expanded_fragment (AST::ASTFragment::create_empty ()),
+      expanded_fragment (AST::ASTFragment::create_error ()),
       resolver (Resolver::Resolver::get ()),
       mappings (Analysis::Mappings::get ())
   {}
@@ -295,7 +295,7 @@ struct MacroExpander
   AST::ASTFragment take_expanded_fragment (AST::ASTVisitor &vis)
   {
     AST::ASTFragment old_fragment = std::move (expanded_fragment);
-    expanded_fragment = AST::ASTFragment::create_empty ();
+    expanded_fragment = AST::ASTFragment::create_error ();
 
     for (auto &node : old_fragment.get_nodes ())
       {

gcc/testsuite/rust/compile/macro41.rs

@@ -0,0 +1,13 @@
+macro_rules! empty {
+    ($($t:tt)*) => {};
+}
+
+empty! {nothing}
+empty! {struct OuterItem;}
+empty! {}
+
+fn main() {
+    empty! {as statement};
+    empty! {any child item};
+    empty! {};
+}